I didn’t notice this before, it might be a new bug. When I resize the browser window to a size that is more narrow than the space allocated by the MetaPDF toolbar, the whole toolbar disappears.
This happens on the Asus flip Chromebook as well, as soon as the screen is rotated so as to read a paper in portrait view. It’s done this for ages, and is quite frustrating. I get round it by not using the Chromebook for annotating/reading which is a shame as it would be very useful otherwise.